1. Field of the Invention
The present invention relates to a mobile communication terminal capable of controlling the transmission power of a base station, and also relates to a transmission power controlling method.
2. Description of the Related Art
In some mobile communication systems, multiple mobile communication terminals that communicate with a base station simultaneously share the same frequency bands. An example of such communication systems is a CDMA communication system. In such a system, a measure has to be taken to prevent the interference among the mobile communication terminals, and “transmission power control,” in which the transmission power of the mobile terminals and that of the base station are controlled, is a specific measure conventionally employed for the purpose. A closed-loop power transmission control, which is a specific example of the transmission power control, includes inner-loop control and outer-loop control.
In the inner-loop control, the mobile communication terminal measures the instantaneous reception quality of a received signal that has been transmitted from the base station, and compares the measured instantaneous reception quality with a target value of the instantaneous reception quality. Then, the transmission power of the base station is controlled using the comparison result. As a result, the reception power of the mobile communication terminal for the signal transmitted from the base station is kept constant, and stable radio communications are accomplished. One of the generally used indicators of the instantaneous reception quality is the signal to interference power ratio (SIR).
In the outer-loop control, on the other hand, the reception quality is measured for a longer period of time than in the case of the inner-loop control. The long-term reception quality thus measured in the outer-loop control is compared with a target value of the long-term reception quality. Then, the target value of the instantaneous reception quality is updated using the comparison result. One of the generally used indicators of the long-term reception quality is the block error rate (BLER).
The length of the period for measuring each of the instantaneous reception quality and the long-term reception quality is normally fixed to a certain value. Such fixation is not suitable for tracking the fading fluctuation, which is a rapid change in the receiving power on the mobile communication terminal side. Such drastic change occurs when the mobile communication terminal moves, or when the radio wave is reflected by a building, or the like. Methods for addressing the fading fluctuation have been proposed by controlling the length of the period for measuring the received signal code power (RSCP) and the interference signal code power (ISCP), both of which is used to measure the SIR. Refer to Japanese Patent Application Laid-open Publication No. 2005-167710.